In the scenario of installing fiber optic cable in dense cable situations, such as in dwelling/housing units, customers typically look for an indoor/outdoor fiber cable that can be used for both aerial drop (from pole to house) or buried/ditch (underground to house) and then can be pulled or pushed through the dense cable situation inside, without being easily attenuated from the rough handling associated with pulling or tugging the cable through the conduits.
Prior art arrangements utilize strengthened loose tube fiber arrangements either using a gel filled tube or a stranded aramid wrapped 4, 6 or 12 fiber assembly. In this case, loose tube refers to the fact that the fibers are arranged loosely within a larger outer sheath or jacket. The fibers can either be basic optical fibers (having only the standard 250 micro UV coating) or “tight buffer” optical fibers (having an additional polymer coating totaling 900 microns over the basic UV coating).
There are many drawbacks associated with the prior art arrangements. First, there are many physical problems with the combination of gel and strength members used in prior art fiber optical cables.
For example, FIG. 1 shows an aerial drop cable having two strength rods, one on either side of the tight buffer optical fiber and a gel for protecting the fibers. The stiff rods are prone to moisture breakdown and fracture, resulting in potential breakage in tight diameter coiling (eg. 6″ diameter). Multiple bends of such cable can also result in pinching or compressing of the central fiber. Cables of this design are ill suited for multiple 90 degree, small diameter, bends in excess of two before GRP (Glass Reinforced Plastic) fracture and then resulting in significant attenuation in the fibers.
Furthermore, the wide flat construction, although acceptable for the aerial drop from the pole to the house or use in ditch leading up to the house, makes it more difficult to navigate through conduits within the apartment building or the house that are tightly packed with other utility cables. Aerial drop cables tend to be too-flexible to be forced through the building conduit and cables using more rigid strength members suffer from the opposite drawback, namely being too sturdy to easily navigate dense conduits. The typical side by side addition of a tone wire (used for detection of buried or hidden cable) in a flat drop cable may also add additional bulk which further interferes with movement through the conduits through the dwelling unit.
Another drawback associated with the prior art is that the semi-coupling of the fibers to the cable jacket by virtue of the gel or stranding (yarn friction with the inside diameter of the jacket) also acts to lock in fiber strain when the cable is installed. Fiber strain is a result of the installation process, where the pulling of the cable also imparts similar stresses to the optical fibers therein.
The gel used in these cables acts to hold the strain in the fibers long after installation. This fiber strain takes time to adjust, over months or even years, so that the fibers can relax relative to the cable jacket. This strain relief or fiber migration may easily result in a slow pulling from the ends of the fiber from their connection points.
As shown in FIG. 2, an installer may typically use loops on the poles to prevent fiber migration within the cables caused by the fiber strain imparted during installation. However, the loops themselves also work to maintain additional unwanted tension in the fibers. This arrangement requires additional cable strength to compensate for the added locked in fiber strain, adding cost and weight to the product, not to mention the use of extra cable adding cost and cluttering the cables on the pole. Further, these lock-in-loops set up a high bending stress scenario for the grp (glass reinforced polymer) rods in the flat drop cable which, when these grp rods are exposed to humidity migration through the jacket over time, begin to loose their integrity and begin to fracture from this coil stress. This results in cable failure and fiber attenuation or breakage.
Yet another drawback associated with the prior art arrangement is that although the gel is good for protecting the optical fibers within the cable during tough installation conditions, the gel used is typically an oil based gel which is flammable. Although a silicone gel is available, it is prohibitively expensive.